1. Field of the Invention
The present invention relates to light-emitting diodes, and in particular, to the structure of electrodes intended for surface mounting of a light-emitting diode which is surface mounted on a circuit board such as a motherboard of an electronic device and emits light sideways.
2. Description of the Related Art
There is a conventional light-emitting diode including a substrate and a light-emitting diode element mounted on the substrate. When the light-emitting diode is soldered to a circuit board or a motherboard of various electronic devices, electrodes electrically connected from the surface where the light-emitting diode element is mounted may be used as soldering terminals. It is known that the light-emitting diode uses through-holes as soldering terminals, even if the light-emitting diode is soldered on the circuit board to emit light in a direction parallel to the surface of the circuit board or the motherboard (for example, see Japanese Patent Application Laid-Open No. 2001-77408).
FIG. 13 shows a cross section of a module that includes a conventional light-emitting diode element that emits light sideways. In this module, the light-emitting diode element 4 is mounted on a substrate 2. Through-holes 10 intended for mounting onto a wiring pattern 8 on a circuit board 6 are formed in the substrate 2. These through-holes 10 are plated to form conductor layers 12 on the inner peripheries, and are filled with solder 14 so that the conductor layers 12 will not produce burrs when cut.
To form the through-holes 10, bores are made in a substrate assembly. The inner peripheries of the bores are plated, and the openings are closed with a resist before the molding of a light-transmitting resin 16. The bores are filled with solder before being cut in the axial direction into a generally semi-cylindrical shape.
The foregoing module of side emission type typically has at least two through-holes 10 in the mounting surface, so as to be electrically continuous with a respective pair of electrodes on the substrate 2 to which the light-emitting diode element 4 is die-bonded and wire-bonded. In order to form the at least two semi-cylindrical through-holes 10 in an identical surface like this, the substrate 2 must have a mounting surface of sufficient size in which to form the plurality of through-holes 10. This has been a hindrance to miniaturization.
Consequently, it has conventionally been attempted to reduce the through-holes 10 in diameter, which has been proved to be difficult in terms of manufacturing. More specifically, in the foregoing conventional module, it has been necessary to close the openings with a resist or a dry film (hereinafter, collectively referred to as a resist) so as to prevent the light-transmitting resin 16 from flowing into the through-holes 10 when sealing the light-emitting element with the resin. Since this resist is intended to close the openings of the through-holes 10, it requires an external dimension greater than the diameter of the through-holes 10. There has also been the problem that the resist can be greatly misaligned from the through-holes 10 when formed on the substrate 2. In view of the diameter and misalignment of the resist, it has been impossible to close the through-holes 10 with reliability unless the resist is formed considerably larger than the diameter of the through-holes 10. The provision of the space for forming the resist increases a dead space on the substrate 2 and results in a great hindrance to miniaturization even if the through-holes 10 are reduced in diameter.
Conductive parts of the substrate 2 may be plated with silver for improved light reflection efficiency. Since the resist and silver have poor adhesion to each other, it is necessary in this case to close the through-holes with the resist before the silver plating of the conductive parts, so as to prevent exfoliation of the resist. Closing the through-holes at one side with the resist in advance, however, makes it harder for the plating solution to get into the through-holes. This requires that the through-holes be increased in diameter so that the through-holes are plated satisfactorily inside. There has thus been the problem of even greater dead space and more difficult miniaturization, when combined with the foregoing space for forming the resist.
Consequently, an attempt has been made to form the through-holes along edges of the mounting surface of the substrate 2, with a sector-shaped cross section derived by cutting a circle into about a quarter. This decreases the contact areas between the substrate and the conductor layers which cover the inner peripheries, however, as compared to the through-holes that have a generally semi-circular cross section. As a result, the conductor layers covering the inner peripheries can sometimes exfoliate from the substrate along with the filled solder and the like.